UC Davis wildlife veterinarian Winston Vickers swabs the ears of a Catalina Island fox. A team of scientists initiated ear mite treatments for the foxes that have dramatically reduced ear mites and signs of ear canal tumors in the animals.

Winston Vickers, a UC Davis wildlife veterinarian, examines a Santa Catalina Island fox for ear mites. A team of scientists led by UC Davis initiated ear mite treatments for the foxes, which have dramatically reduced ear mites and signs of ear canal tumors in the foxes. Julie Lynn King/Catalina Island Conservancy

A male fox is seen in Banning Meadow on Catalina Island. In 1999, there were only about 100 Catalina Island foxes left. Their numbers are now up to more than 1,700.

Fox pups nuzzle in the ground at Quail Valley on Santa Catalina Island. UC Davis scientists and their island partners found alarming rates of ear mites and ear canal tumors in the foxes. Ear mite treatments they began have since dramatically reduced the problem, their studies show. Julie Lynn King/Catalina Island Conservancy

The Catalina Island fox, one of the world’s most endangered species, suffered from frequent tumors for the past decade until scientists discovered a relatively simple solution: treating ear mites.

New research released by veterinarians at UC Davis and the Catalina Island Conservancy shows that treating ear mites with an antiparasitic drug called ivermectin helps block the formation of cancer. The incidence of ear infections that can lead to cancer were reduced from 98 percent to 10 percent among foxes that received the drug during a six-month pilot program as part of a recent study.

The brink of extinction

Efforts to save the foxes, one of the six subspecies of the island gray fox native to the Channel Islands, began in the late 1990s when residents noticed a decrease in their numbers. The fox population noticeably declined on several of the Channel Islands because of canine distemper brought by island visitors.

The disease killed roughly 90 percent of the island fox population; as of 1999, roughly 100 foxes remained, said Winston Vickers, associate veterinarian with UC Davis and lead author of the study. As a result, all fox populations on the island were listed as endangered under the federal Endangered Species Act.

In 2000, an effort to bring back the population prompted the Catalina Island Conservancy and the Institute for Wildlife Studies to create the Catalina Island fox recovery program, consisting of captive breeding, vaccinations for distemper and close observation of the remaining foxes.

Ear mite study results

As scientists watched the recovering fox population, they noticed the prevalence of ear mites and, more alarmingly, the incidence of cancer.

A pilot study spearheaded by the conservancy began with 100 foxes to determine whether ivermectin would be effective. A tiny dose of 0.5 millimeters of the antiparasitic drug is administered through a syringe slid into the animal’s ear canal.

“It’s an oily based treatment that coats the inside of the ear and kills the mites,” said Calvin Duncan, wildlife biologist for the Catalina Island Conservancy and co-author of studies.

Initial lumps that indicate early stages of most tumors are no longer being seen, Vickers said.

Most of the fox deaths due to ear tumors were found in older animals that already had secondary infections that reduced their overall fitness, scientists say. Vickers said there is not enough data to determine how many foxes have died from cancer.

He said another study is underway comparing the genetics of foxes with the infections and those without.

About 400 foxes have been captured and given health assessments as part of the conservancy’s yearly trapping survey.

“At this point we’re not planning to expand treatment,” said Duncan, who has applied treatment since 2006 and has seen success in preventing other foxes from catching mites, especially among pups.

The next step is evaluating ear assessment data on the full-scale treatment that has been done.

Efforts are working

In 2014, the fox population was surveyed at 1,717, and Duncan said this year’s data should indicate similar numbers.

He added the overall population has exceeded its historical numbers.

The numbers have rebounded so much that the Catalina Island fox may no longer be listed as an endangered species; this designation, which prohibits hunting of the animals and other restrictions, is under review by the U.S. Fish and Wildlife Service.

Threats will always be present, he said, referencing an increase in foxes trapped in trash cans and hit by cars. The conservancy will continue to support, monitor and protect the endemic mammal, he said.

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